Numerous types of proportional pneumatic flow control valves are presently available which accurately control flow over a wide range of flow control applications. These single valves are typically calibrated to a specific range of flow rates; the accuracy of these valves is dependent upon the total range of capable flow, which is otherwise known as the “Full Scale” of the valve. A problem is that these units generally do not offer precision flow at lower rates, the valve percentage of error for the full scale of the valve may be too great; and, the low flow imprecision can be too problematic to the application.
Heretofore, there is known means to proportionally control the flow rate of a medium with flow control valves that increase accuracy over minimal flow rates and that allow for higher flow rate performance. For example. U.S. Pat. No. 5,462,253 to Asthana et al. discloses a dual slop flow control valve which operates by incorporating an electronically controlled valve actuator. The actuator subsequently controls a valve assembly, this process is accomplished using two mechanical valves in sequence.
Another common approach is to use two independent valves in parallel or in sequence to increase accuracy over particular segments of the full flow range. One valve is typically used for lower flow rates, and then a larger valve subsequently compensates for the higher flow rates. Both valves are independently controlled.
There is a need for a mechanically simplified, low-cost device that can increase the precision of low flow output during the initial flow performance of a device's full-scale flow capability while also allowing for higher volume flow for a remaining portion of the full-scale output.